In the formation of photographic paper it is known that a white pigmented layer is placed directly under the photosensitive silver halide emulsion. The white pigmented layer is typically a polymer such as polyethylene or polyester in which a white pigment such as TiO.sub.2 is dispersed. Such a layer is highly reflective and opaque and enhances the image sharpness of the exposed and developed image. Furthermore it is known in the art that as the concentration and amount of TiO.sub.2 in the reflective layer under the emulsion is increased that the image sharpness is increased. Sharp images are highly desirable and have significant commercial value.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene sheets laminated to cellulose photographic paper for use as a reflective receiver for the thermal dye transfer imaging process. In the formation of biaxially oriented sheets described in U.S. Pat. No. 5,244,861, a coextruded layer of polypropylene is cast against a water cooled roller and quenched by either immersion in a water bath or by cooling the melt by circulating chill liquid internal to the chill roll. The sheet is then oriented in the machine direction and in the transverse direction. The biaxially orientation process creates a sheet that has a highly pigmented layer on the topside of a voided layer. The pigmented layer provides a highly reflective layer immediately under the image layer. There remains a need to create an image-sharpening layer that provides additional sharpness to a photosensitive silver halide layer without having to additional expensive white pigment such as TiO2. While TiO2 is highly reflective and desirable, it is very expensive and furthermore tends to scatter light in multiple directions, which tends to corrupt the purity of the photosensitive dyes.
In U.S. Pat. No. 5,866,282 it has been proposed to use biaxially oriented polyolefin sheets laminated to photographic grade paper as a photographic support for silver halide imaging systems. In U.S. Pat. No. 5,866,282 numerous advantages are obtained by the use of the high strength biaxially oriented polyolefin sheets. Advantages such as increased opacity, improved image tear resistance and improved image curl. While all of these photographic improvements are possible with the use of biaxially oriented polyolefin sheets, the use of biaxially oriented sheets with solid surface skins for silver halide imaging systems is restricted to the amount of TiO.sub.2 that can be dispersed in the polyolefin polymers as well as to the practical limitation of the thickness of the pigmented and voided layers.
In U.S. Pat. No. 5,888,681 it is disclosed that a photographic element with a microvoided base has some very beneficial attributes in providing an image with an opalescent appearance. This invention is substantially free of TiO.sub.2 to take full advantage of the unique voided layer. While this is highly desirable in certain markets and the images are sharp and pleasing, the overall sharpness is not as high as it could be if TiO.sub.2 was used in a layer under the light sensitive emulsion. Unfortunately when more substantial amounts of TiO.sub.2 are added, the opalescent effect is diminished. There remains a need to provide additional sharpness in an imaging print with minimal use of TiO.sub.2.
In U.S. Pat. No. 5,429,916 and U.S. Pat. No. 5,466,519 it is disclosed that a multi layers of polymer applied to a base sheet in which the upper layer of polymer is below the silver halide emulsion and also contains more TiO2 than lower layer of polymer. In both these cases the TiO2 is below the silver halide layer and is used for opacity and sharpness. There remains a need to provide addition sharpness to an imaging print without the further addition of expensive white pigment below the photosensitive layer.